1. Field of the Invention
This invention relates to softened wood pulps with good absorption properties, and a process for making such pulps.
2. Description of the Related Art
It is desirable for many industrial applications to produce a cellulosic wood pulp which maximizes both softness and absorbency. The softness of a pulp product is greatly influenced by the degree to which the constituent wood pulp is debonded, i.e., the extent to which hydrogen bonds within the wood pulp are broken; softer pulps and pulp products typically having decreased hydrogen bonding. Wood pulp softness can be expressed in terms of properties such as Mullen strength (the strength of pulp or a pulp product, measured in kilopascals (kPa), defined in greater detail below), and Kamas energy (the energy required to convert a given amount of pulp or pulp product to a fluff material, measured in watt hours per kilogram (Wh/kg), defined in greater detail below). Lower values of Mullen strength and Kamas energy correlate to softer, increasingly debonded, pulp.
Many industrial pulp applications involve the conversion of pulp to fluff pulp by mechanical means. Fluff pulp has the inherent characteristics of bulk, softness, high absorbency, and resiliency. Resiliency often depends on the length, diameter, and stiffness of the pulp fibers. Long, stiff fibers will provide greater bulk and resiliency than short, flexible fibers due to their relatively larger interfiber distances to compaction. The inter-fiber voids formed in fluff or debonded pulp determine to a large extent the absorbency of the pulp. Large void areas lead to higher absorbency since it is these void areas that hold moisture.
The efficient mechanical fluffing of wood pulp requires a pulp that will debond to a desirable degree with minimum power input and little mechanical fiber damage. Such pulp must have the proper bulk and degree of inter-fiber bonding. A hard pulp sheet will increase the power needed to create fluff pulp and will therefore lead to increased fiber damage. An unduly soft pulp sheet will lead to pull-out of large pieces of pulp, causing poor fluffing.
It is known that the use of cationic surfactants in the manufacture of wood pulp products, for instance sanitary papers, yields a product which has a soft hand feel. This is accomplished through the lubricating nature of the substantive softening molecule; less extensive inter-fiber bonding leading to greater bulk and the plasticizing effect of these additives. Cationic surface-active agents are used in the manufacture of fluffed debonded pulp to increase bulk, reduce Mullen strength (and hence, reduce inter-fiber bonding), and impart softness to fibers. The lubricating effect of these agents prevents extensive inter-fiber bonding and increases the bulk of a machine formed pulp sheet. In fluffing, such agents improve the debonding characteristics of a pulp sheet. This results in lower power requirements and less fiber damage. Reduced fiber damage produces a fluffed pulp with better bulk and resiliency.
In the manufacture of fluff or debonded cellulosic pulp, cationic surfactants are used primarily to reduce the inter-fiber bonding of pulp sheets. Reduced inter-fiber bonding is normally associated with a significant reduction in Mullen strength. Since a significant amount of energy is required to convert pulp to final fluff product, the use of debonded pulp reduces overall energy costs of conversion.
It has long been accepted in the paper making industry that pulp softening and debonding cannot be accomplished utilizing cationic surfactants (or even nonionic debonders which enjoy limited use) without sacrificing absorbency properties of wood pulp. It is generally believed that debonding pulp with hydrophobic materials, such as cationic surfactants, results in the reduction of absorbent properties. Reductions in absorbent properties using standard cationic quaternary ammonium compounds for debonding can be quite substantial (e.g., 18% reductions for a partially debonded southern bleached kraft pulp and about 27% reductions for a fully debonded southern bleached kraft pulp).
There are four cationic chemical materials used to soften pulp to produce a fluff or debonded pulp. All of these materials are quaternary ammonium compounds typified by a nitrogen ion attached by covalent bonds to four organic groups. An anion, usually a halide (e.g., a chloride) or sulfate group, is associated with the positive ion of the quaternary nitrogen. Examples of such quaternary ammonium compounds include the following generic structures: ##STR1##
In the above drawings R is typically a C-12 to C-18 alkyl group or a C-9 aryl group, as appropriate. X is typically a halide or sulfate ion. Values for n range from 2-30.
The above-depicted cationic surface-active softening and debonding agents can be supplied as liquids, pastes, powders, solutions in water and alcohol, or solutions in water alone. However, quaternary ammonium debonding and softening agents are generally applied as dilute emulsions of water. Addition of a highly diluted emulsion is preferred since this assures uniform distribution of the debonder.
The range of surfactant treatment rates required for use as a debonding agent is usually between 3-10 pounds per ton of pulp. The range of treatment rates required for a softening agent is usually between 1-6 pounds per ton of pulp.
As one skilled in the art would recognize from reviewing the above-depicted chemical structures, there are many species of quaternary ammonium materials which can be used to improve softness and debond wood pulp. There are advantages and disadvantages to each type. However, the use of any type of quaternary ammonium compound to soften or debond cellulosic wood pulp uniformly has the disadvantage of adverse effects on absorbency.
Nonionic agents are also used to a limited extent to debond pulp in the paper industry (e.g. Berocell 587, Eka Nobel) but even they cause adverse affects on absorbency. It is believed that this effect is due to the presence of long hydrophobic side chains.
Accordingly, it would be desirable to provide a method of treating pulp to form fluff pulp with improved bulk, softness and reduced inter-fiber bonding without sacrificing the absorbent properties of the pulp.